DLR researchers in Stuttgart have become the first team in the world to demonstrate the feasibility of the free-piston linear generator, which they accomplished using a test bench developed specifically for this purpose.

A particularly powerful mechanism in combination with a highly dynamic feedback control system controls the individual components of the free-piston linear generator (FKLG) – the internal combustion component, linear generator and gas spring.

Scientists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have developed an entirely new kind of drive concept to extend the range of electrically powered vehicles. The free-piston linear generator (Freikolbenlineargenerator; FKLG) is a combustion engine that generates electricity. This electrical power drives the electric car when its battery is flat. In contrast to conventional range extenders, different fuels can be used in the free-piston linear generator.

Researchers at the DLR Institute of Vehicle Concepts in Stuttgart have demonstrated the feasibility of this technology on a test bench specifically developed for this purpose. This makes them the first in the world to succeed in commissioning this kind of energy converter, comprising an internal combustion component, a linear generator and a gas spring. "Innovative solutions like the free-piston linear generator will help to make electrically powered mobility an everyday reality and demonstrate the scientific strengths of Baden-Württemberg as a location," said Rolf Schumacher, the Ministerial Director at the Ministry of Finance and Economics in Baden-Württemberg, at the official inauguration ceremony for this project.

New structural approach with powerful feedback control

Engineers have been aware of the principle of this drive unit for some time. Through the installation of a gas spring, DLR researchers have now succeeded, for the first time, in operating this system in a stable manner. The challenge here was to develop a particularly powerful mechanism with a highly dynamic control unit that regulates the complex interactions between the individual components," said Ulrich Wagner, DLR Director of Energy and Transport, as he described this innovation.

The free-piston linear generator works in a similar manner to a conventional combustion engine. But instead of converting the linear movement of the piston into the rotational movement of the crankshaft, it generates electricity directly. A fuel-air mix is ignited in the combustion chamber. This expands and pushes the piston towards the gas springs. These springs decelerate the piston movement and push it back. The linear generator converts the kinetic energy of the piston into electricity and this in turn powers the electric motor. The control system devised by the DLR engineers is able, for example, to control piston movement accurately to within one tenth of a millimetre. At the same time, it recognises fluctuations in the combustion process and compensates for them.

As a new kind of range extender, the free-piston linear generator (FKLG) makes electric vehicles more flexible

Optimum operating strategy through variable properties

In contrast to conventional drive technologies, the free-piston linear generator enables the compression ratio, piston speed and cubic capacity to be adjusted flexibly. For this reason, different fuels can be used – from petrol, diesel and natural gas through to ethanol or hydrogen. By virtue of its versatile properties and depending on vehicle speed and driving characteristics, the settings of the DLR range extender can always be adapted to deliver the optimum operating strategy. "We can therefore set the operating point of the engine when driving to ensure that we can drive as efficiently as possible and in the most environment-friendly manner," summarised the Director of the DLR Institute of Vehicle Concepts, Horst E. Friedrich. At the same time, the free-piston linear generator functions with fewer components. For example, certain crankshaft and camshaft components normally essential in a conventional combustion engine can be dispensed with altogether.

Making electric cars more flexible

Exceptionally efficient range extenders such as the free-piston linear generator are more than just emergency power units. The free-piston linear generator makes it possible to equip electric vehicles with a much smaller battery while still deriving optimum benefit from electrically powered mobility. Short distances of up to 50 kilometres, in town for example, can be covered using only electrical power; for longer distances, the range extender takes over. It provides the accustomed peace of mind and autonomy of a combustion engine. As a bridging technology, it makes electric vehicles an attractive option for the general public.

Technology transfer into industry

"With our functional demonstrator, we have shown for the first time that our free-piston linear generator principle can be implemented. In the next step, we need to work with industry to develop this technology and build a prototype," explained Friedrich. To accomplish this, DLR has concluded a technology transfer contract with Universal Motor Corporation GmbH and will provide scientific support during further work. One of the tasks ahead is to optimise the weight and size of the free-piston linear generator in such a way that one or more of the assemblies can be located in the underbody area of a vehicle. In this way, initial estimates suggest that an additional range of about 600 kilometres could be achieved without increasing the weight of the car.

31 Comments

I can think of a variation of this engine if anyone from DLR would like to engage in a friendly interesting chat...

Also... How much consideration and work has gone into using a Sterling engine as a range extender..

21/02/2013 09:22 -
from M. Trovatello, DLR Web Portal Team

Dear Paul,

thanks for your comment. Prof Friedrich is the person in charge, see above. You may contact him directly via mail form by clicking on his name. Best, Marco

22/02/2013 17:48 -
from Berto Aussems

Exellent research, gratulations! This is a great unit to run in my basement. It's working quet and it is powerfull enough to care for all my home energy needs. But many parts are still concept. Hurry up pls.

18/03/2013 20:28 -
from Antonio Calabro'

Congratulations !This is a great research, a bridge from the two parallel world that, in normal opinions, will never cross each other.Ad MAiora

AntonioItaly

22/03/2013 22:05 -
from Martin Schuermann

I wonder how this engine compares with fuel cells in terms of efficiency and reliability.I would like to test this engine in one of our trucks as a range extender, running on hydrogen.www.visionmotorcorp.com

25/03/2013 00:23 -
from Michael

This is an exciting development of opposed cylinder design. I assume that you will need one generator per cylinder? You can certainly minimize the valve train with this.

Any thoughts of a direct injected two stroke? It seems that this is optimized for a two stroke cycle- installing central valve would greatly weaken the engine, and increase the complexity.

Assuming that it is two stroke, part of the gas shock charge could be used for supercharging, especially useful for that cycle.

Congratulations!

25/03/2013 14:38 -
from Kevin Sorbello

The question that has yet to be answered is: what is the overall efficiency of the linear generator? In other words, what is the actual electrical energy generated for the potential fuel energy put in?

28/03/2013 02:32 -
from C W dierkes

Similar concept to the hydrostatic INGOCAR.

04/04/2013 05:30 -
from S.Seetharaman

Congrats to the whole team. Really interesting.Regards,S.Seetharaman

05/04/2013 10:32 -
from R. Smallegange

Congratulations on the great achievement to have a functional and controllable free piston engine. To get the efficiency of the linear generators to a acceptable level may prove to be a very difficult job but if you succeed, you might have a potential winner.Good luck in your development and all the best wishes from an other opposed piston engine Range Extender company.Best regards from R. Smallegange (www.peecpower,com)

21/04/2013 01:46 -
from Goltermann

Very impressive concept. It begs two questions?

Can it be scaled up? I'm thinking ~100kW per unit instead of 35kW. For trucks, buses and maybe marine applications?

How is it cooled? The animation video doesn't reveal the cooling principle.

24/04/2013 12:00 -
from Muthukumaran

It is intresting concept, how about the induction of engine, is it super or turbo chrged?. How much energy does it produce, and what about the cooling of it?.

24/04/2013 14:04 -
from Florian Kock, FPLG project manager, DLR

Thank you for your comments. I am going to provide some more information concerning your questions:

- Upscaling: Upscaling to a greater power output is possible in two different ways. Firstly, the swept volume per cylinder can be increased. Secondly, it is possible to use several units. This is the equivalent to today’s multi-cylinder engines – but in contrast to those, some of the cylinders / units of the FPLG can be deactivated completely in case they are not needed currently.- Cooling: Water cooling for combustion section, linear generator and gas spring with different temperature levels for the 3 subsystems.- Gas exchanged: Indeed, it is necessary to turbo- or supercharge the engine making sure the scavenging process works satisfactorily.- Power output: Our current function demonstrator generates up to 12 kW from a single piston module operated at a frequency of 20 Hz. For a production version the frequency will be increased and a layout with two pistons will be used. We expect an electrical power output between 20 and 35 kW per module to be most beneficial.

31/05/2013 12:31 -
from Claudiu Sav

Every time when I see this kind of system I think about one thing: mechanical advantage, actual the lack of it. You need more magnetic circuit and permanents magnets than on a "regular" system with crankshaft, gearbox and generator/alternator. It's a more elegant design but also more expensive.

08/06/2013 13:43 -
from Christopher Haase

Considering the footprint, multi-fuel use and efficiency of this design has anyone considered how important this engine could be in cogeneration, off grid and emergency power. I have more than a few volunteer projects that would be solid testing grounds to prove this engines ability to help save lives and reduce home energy costs.

11/06/2013 20:19 -
from Dick Duffy

I'm curious, it appears natural gas can be used based on what I've read. I would be interested to learn if you've tried using a gaseous fuel on your proof-of-concept prototype. As you probably have guessed, I'm a CNG proponent, but I firmly believe clean-burning CNG - especially when used in a steady state application such as your linear generator - can meet driver range requirements when combined with electric propulsion. It might be the best of both worlds as far as clean vehicles are concerned. Of course Hsub2 would be even cleaner, but must be made not simply extracted and treated like natural gas.If you need someone in the US market, let me know when the time comes. Thank you, Dick

23/06/2013 21:45 -
from Michiel

Could you give an estimation of when first units are available for third party proof of concept?

07/07/2013 17:40 -
from sathya

How much electricity produced by the linear generator????what is the mileage given by these kind of vehicle???what is the main advandage for this?

17/07/2013 19:09 -
from Peter Barnes

Any idea of the conversion efficiency of this engine system?

22/07/2013 22:37 -
from Kechik

Please let us know when and where we can purchase a working unit for prototyping. Congratulations!

01/08/2013 10:40 -
from Alex Heron, FPLG research assistant, DLR

Thank you for your intrest and the questions, here is some additional information:

- General: With “power output” we understand the electricity extracted by the linear generator. Any information we created for the mileage of vehicle concepts are estimates in regards to the vehicle (e.g. Battery size) in combination with estimates in regards to the FPLG technology. Therefore we are cautious to say more than that there is potential in the technology. - Potential: As correctly noted the FPLG has a slightly larger generator then a rotating system, which is a disadvantage. Yet it does not need a crankshaft and with a central combustion chamber it can also lose the camshaft and valves. The FPLG design leads to further promising potentials, which are or will be described in detail in research papers. It is not a single potential which makes the technology interesting but the combination of the multiple potentials.- Alternative use: We have in mind that our concept has a wider span of application. We have had contact to firms in the field of cogeneration and presented a paper on the topic.- Alternative fuels: For the demonstration of feasibility we used gasoline as fuel. As a gaseous fuel would need a separate fuel supply we did not test alternative fuels yet. This adaption is possible and an attractive option. Concerning market or cooperation issues, I would be glad to be contacted via e-mail (alex.heron@dlr.de).- Prototype: The German Aerospace Center does not aim to create units for sale. We are focused on the research necessary before pre-production development begins. Therefore a spin-off company was founded for acquisition of funds to support our research and commence with pre-production development.- Efficiency: The functional demonstrator is not optimized for efficiency. Yet it gives us a better understanding to base our simulations and estimates on. Our results will be published in near future.

Kind regards,Alex Heron

03/09/2013 12:41 -
from Mr. Pendelschwanz

At last! What else can be said. I just hope this amazing thing will get permission to exist.

But, what if I do not want to move entire automobile by my side, I want more lightweight transportation. More lean, less fat. Some kind of 3-wheeled «road boat». Then I need mini-version of this thing, say 5 kWt one.

23/10/2013 11:10 -
from José García

Congratulations!Alex says that have not been tested with gas but the specs say it can operate with Natural Gas and Hydrogen. Express different compression ratios to various fuels refernte but not Hydrogen.questionsThat compression ratio calculated applying?That minimum gas pressure required for admission?Could work with Oxyhydrogen gas?Greetings. José García-Spain-

24/10/2013 12:12 -
from José García

I forgot to ask the consumer.How many liters per minute or cubic meters per hour needed to produce 35KWJosé García-Spain-

29/10/2013 16:39 -
from Florian Kock, project manager FPLG

Thanks again for this long-lasting interest. Here are the answers concerning the recent questions:

5kW-Version: The FPLG technology itself is not limited to a certain power range. As it is the case with conventional engines, very different sizes are feasible with the same basic technology. DLR activities aim primarily to a 25- to 35-kW version, but a smaller FPLG is far from being out of scope.

Gaseous fuels: The FPLG is able to adapt to different liquid and gaseous fuels due to its variable compression ratio. This variability has been demonstrated on the test bench. However it has not yet been 'used' for burning different fuels. The working principle of the FPLG generally allows for the use of a wide range of fuels (including hydrogen). For a practical realization some auxiliaries, for example the fuel injector, will have to be implemented as necessary.

Litres per minute: In a recent publication (http://link.springer.com/article/10.1007/s35146-013-0229-9 or available as print) some efficiency values are published. As these numbers are only to be understood with some background information, I am not quoting single numbers here: But please feel encouraged to have a look at the article. Knowing gasoline's density and its calorific value, a hand-held calculator will help you to convert the efficiency [percentage] to any unit like "litres per minute" or "cubic metres per hour" :)

17/03/2014 04:31 -
from Markus

congrats .. its the right way

14/05/2014 21:57 -
from CHANDRA NAGESWARA PRASAD BOLLA

Sir I am a mechanical 2nd year student in INDIA It will be not real-able for you that even I have got the idea which nears to your concept 6 months back when my lecturer teaching me a lesson about I.c engines by showing the engine, That nearly 12% of the total energy is going to dispeats out in the form of heat and combustion in I.c engine is progressive So, The first few layers below the plug will not show much effect on the pistion located at the bottem It will be efficient when we place a piston at the top too .this is going to meet high compression ratios In diesel engines this will increase in efficiency Iam trying to develop a base for my concept and trying to design a engine prototype thank you for giving an opportunity to write the review

21/05/2014 18:03 -
from Dr. Kenneth Noisewater

This is pretty excellent, and short of SOFC is what I would want in a serial hybrid. However, how do you handle exhaust gas? It looks like a 2-cycle, which I suppose means you run EGR from one motor to the other and back to get exhaust cleaner?

Also, please fit a turbine on the final exhaust in order to recuperate power from its flow, basically like a turbocharger but with a generator instead of an intake air impeller. And after that, maybe even a turbosteamer like BMW's :)

25/08/2014 13:08 -
from John Burns

What is the status of this engine? Many engines have been given by the press as the next great thing to disappear to the ether. That is not to say they were feasible of course.

Will this engine be seen in a production car as a range extender? Mazda, and others, are working on Wankels as range extenders. How will this compare?

07/09/2014 18:44 -
from Maurice Tardif

Do you plan to make an external generator base on your extender, specialise for 100% electric car recharge in case of power outage.

Or for charge a big battery pack for home, or other power outage solution.

12/07/2015 09:55 -
from Stuart Reid

Congratulations in achieving the the dynamic control of the oscillating pistons. It appears that you can ensure the correct timing with respect to the ports and also the variation in compression ratio while using the generator as a starter motor. And that all these can be monitored and altered as required in a split second. That's a great milestone.

Combustion: You will always have to keep abreast of the clean air requirements and a key performance issue is the combustion process. I am not sure whether it has been contemplated but we often think only in terms of SI or CI ignition. It seems to me that the addition of a corona discharge, even in a diesel engine, may eliminate some of the CI lag and give better control over combustion with respect to all types of directly injected fuels. A further improvement may be to move away from the present point injection system which has become a tradition. I have some ideas regarding a disc injector which allow fuel to enter the cylinder from right around the perimeter of the cylinder through the corona. If you would like to discuss this further, I should be happy to share the concept with you.